The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Horseshoe crabs are arthropods, in the same subphylum as arachnids (including spiders). All four living species live in shallow seas with muddy or sandy bottoms where they forage for worms and mollusks. Shown here is the Atlantichorseshoe crab (Limulus Polyphemus). Multiple light sensitive organs dot its carapace and telson (rigid tail), which serves to right the animal when overturned by predators. Gills are located behind their legs. Horseshoe crab blood has hemocyanin as an oxygen carrier rather than hemoglobin (they appeared before hemoglobin evolved), explaining the blue color of their blood.

As shown in the photo, females are substantially larger than males. During breeding season, females deposit several thousand eggs at a time in soft mud or fine sand at the ocean’s edge, the following male immediately fertilizing the eggs. Most eggs and the resulting larvae are eaten by other shoreline organisms, but enough survive through 6 molts in 12 months to become adult horseshoe crabs and continue the species. At least it’s worked for the past 450 million years!